Swirl Nozzle Research Articles

Experimental study on effects of surfactant and spray inclination on heat transfer performance in nonboiling regime

ABSTRACT The present research emphasizes the optimization of surfactant concentration and examines its effect on spray cooling heat transfer using a pressure swirl nozzle with a 0.51 mm orifice size. Three distinct surfactants, namely, polyvinyl pyrrolidone (polymer surfactant), Sodium lauryl Sulfate (an anionic surfactant), and Tween20 (a nonionic surfactant) have been mixed in distilled water at different volume fractions. The optimum concentration levels of these surfactants have been identified experimentally and result analyzed for surfactant systems revealed that nonionic and anionic surfactants provided higher cooling compared to polymer surfactant that diminished the heat transfer capacity of distilled water. It was also observed that temperature nonuniformity did not improve with any of the surfactants and a maximum heat transfer coefficient of 32⨯103 W/m2K was attained with Tween20 at optimum concentration. The heat transfer performance degraded beyond the optimal concentration due to bubble formation. In addition, the effect of spray inclination of 30º to lessen the effect of foaming and bubble accumulation at high concentrations has been examined. The heat transfer coefficient increased by almost 13% when compared to horizontal spray, reaching a maximum heat transfer coefficient of 36.1×103 W/m2K with Tween20, and the optimum concentration back shift phenomenon was observed. Experiments performed in the present study will not only give insights into the understanding of the spray cooling enhancement, but also facilitate the designing of models for industrial applications.

Impact of a Centerbody on the Unsteady Flow Dynamics of a Swirl Nozzle: Intermittency of Precessing Vortex Core Oscillations

Abstract The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles. This is caused by the presence of a marginally unstable hydrodynamic helical mode that induces precession of the vortex breakdown bubble (VBB) around the flow axis. The PVC can impact emissions and thermoacoustic stability characteristics of combustors in various ways, as several prior studies have shown. In this paper, we examine the impact of centerbody diameter (Dc) on the PVC in a nonreacting flow in a single nozzle swirl combustor. Time-resolved high-speed stereoscopic PIV measurements are performed for combinations of two swirl numbers, S = 0.67 and 1.17 and Dc = 9.5 mm, 4.73 mm, and 0 (i.e., no centerbody). The bulk flow velocity at the nozzle exit plane is kept constant as Ub = 8 m/s for all cases (Re∼20,000). The centerbody end face lies in the nozzle exit plane. A new modal decomposition technique based on wavelet filtering and proper orthogonal decomposition provides insight into flow dynamics in terms of global modes extracted from the data. The results show that without a centerbody, a coherent PVC is present in the flow as expected. The introduction of a centerbody makes the PVC oscillations intermittent. These results suggest two routes to intermittency as follows. For S = 0.67, the VBB and centerbody wake recirculation zone regions are nominally distinct. Intermittent separation and merger due to turbulence result in PVC oscillations due to the destabilization of the hydrodynamic VBB precession mode of the flow. In the S = 1.17 case, the time averaged VBB position causes it to engulf the centerbody. In this case, the emergence of intermittent PVC oscillations is a result of the response of the flow to broadband stochastic forcing imposed on the time averaged vorticity field due to turbulence.

Influence of swirl intensity on atomization characteristics and dust-reduction using pressure swirl nozzle

ABSTRACT Pressure swirl nozzles are widely applied in various mineral industries, and their excellent spray performances have been found to be directly related to dust removal results. In this study, in order to examine the influencing effects of the swirl intensity levels of pressure swirl nozzles on atomization characteristics and dust-reduction efficiency, five spiral guide grooves were added for the purpose of improving the quality of the atomization and dust control. The swirl intensities of pressure swirl nozzles were first theoretically investigated using the established models. The effects of the swirl intensities on the atomization angles and distributions of the atomization sizes were comparatively studied using theoretical calculations, numerical simulations, and experimental methods. A dynamic wind tunnel was built in order to conduct dust-reduction experiments with different swirl intensities. The proximate and ultimate analysis results and the sizes of the coal samples were characterized, and the mechanism of the dust reduction was summarized. The results demonstrated that the applications of spiral guide grooves improved the atomization angles and coverage areas by up to 98.8° and 73%, respectively. At the same time, the enhancements of the swirl intensities were observed to reduce the uniformity index of the distribution sizes,` which significantly decreased the Sauter mean diameter (SMD) from 101.3 μm to 22.1 μm. The atomization results revealed that the increasing of the stagger angles benefited the larger atomization angles and smaller SMD. In addition, the proposed theoretical model had achieved better prediction accuracy. Also, the high atomization quality had obviously improved the dust-reduction ability of the pressure swirl nozzles, with the efficiency of total dust and respirable dust removeable increased by approximately 11.23% and 9.33%, respectively.

The Role of the Centerbody Wake on the Precessing Vortex Core Dynamics of a Swirl Nozzle

Abstract The precessing vortex core (PVC) phenomenon in swirling jets is a helical instability in the flow driven by the coherent precession of the vortex breakdown bubble (VBB) around the flow axis, resulting in the helical rollup of the shear layer. This instability is driven by flow processes in the region upstream of the VBB. Centerbodies, commonly employed in combustor nozzles, create a centerbody wake recirculation zone (CWRZ) that can interfere with VBB precession and hence suppress the PVC. We study this phenomenon in a swirl nozzle with a centerbody whose end face is flush with the nozzle exit plane, using large eddy simulations (LES) and linear hydrodynamic stability analysis for flow Reynolds numbers Re = 48,767 and 82,751, based on nozzle exit diameter and bulk flow velocity. For one of the Re = 82,751 cases, the centerbody end face diameter is halved, resulting in the onset of coherent VBB precession. Linear stability analysis reveals a marginally unstable mode in this case. The same mode is found to be stable in the nominal cases. Structural sensitivity analysis shows that the VBB precession eigenmode is sensitive to changes in the time-averaged flow in the VBB-CWRZ merger region. This suggests that the reduction in CWRZ length due to halving the centerbody end face diameter is the reason for the onset of VBB precession. These results suggest that in general, spatial separation between the CWRZ and VBB can result in the onset of VBB precession and the emergence of PVC oscillations in flows with swirl.

3D-Printing Enables Fabrication of Swirl Nozzles for Fast Aerosolization of Water-Based Drugs

Portable inhalers are used for delivering drugs to the lung in the form of aerosols and form the standard treatment for diseases such as Asthma, COPD, and cystic fibrosis. However, for aqueous drug formulations, spray nozzle chips have so far been restricted to cleanroom manufacture due to their small feature sizes. Here we present a spring-actuated 3D-printed swirl nozzle that sprays an aqueous drug solution, matching propellant-containing inhalers in aerosolization time. The use of two-photon polymerization enables the small nozzle feature size of 100 $\mu \text{m}$ and device print times of only 4 min, making serial mass-fabrication a viable option. Our 35 bar spring-operated swirl nozzle prototype achieves mean volumetric particle sizes of 12.5 ${\mu }\text{m}$ on doses of 100 ${\mu }\text{l}$ , aerosolized in 270 ms, as fast as a propellant-driven inhaler. [2021-0002]

Numerical Simulation and Experimental Study on Flow Field in a Swirl Nozzle

In order to study the internal flow characteristics and external droplet velocity distribution characteristics of the swirl nozzle, the following methods were used: numerical simulations were used to study the internal flow characteristics of a swirl nozzle and phase Doppler particle velocimetry was used to determine the corresponding external droplet velocity distribution under medium and low pressure conditions. The distributions of pressure and water velocity inside the nozzle were obtained. Meanwhile, the velocities of droplets outside the nozzle in different sections were discussed. The results show that the flow rate in the swirl nozzle increases with the increase in inlet pressure, and the local pressure in the region decreases because of the excessive velocity at the internal outlet section of the swirl nozzle, resulting in cavitation. The experimental results show that under an external flow field, the minimum droplet velocity occurs in the axial direction; starting from the axis, the velocity first increases and then decreases along the radial direction. Swirling motion inside the nozzle and velocity variations in the external flow field occur under medium and low pressure conditions. The relationship between the inlet pressure and the distributions of water droplets’ velocities was established, which provides a reference for the research and development of the swirl nozzle.

Effect of the Slot Number on the Breakup and Atomization of Liquid Film in Swirl Nozzle

Effect of the Slot Number on the Breakup and Atomization of Liquid Film in Swirl Nozzle

Numerical study of bubbly flow in a swirl atomizer

In this work, we extend our previous research on swirl nozzles by introducing bubbles at the nozzle inlet. A large-scale hollow cone pressure-swirl atomizer is studied using scale-resolving simulations. The present flow conditions target a Reynolds number range of 600 ≤ Re ≤ 910 and gas-to-total volumetric flow rate ratios between 0.07 ≤ β ≤ 0.33 with β = 0 as an experimental and computational reference. The computational setup has relevance to high-viscosity bio-fuel injection processes. The flow rate ratio and bubble diameter sweeps are carried out to study their effect on the inner-nozzle flow and the liquid film characteristics outside the nozzle. The present flow system is shown to pose highly versatile physics, including bubble coalescence, bubble–vortex interaction, and faster liquid film destabilization relative to β = 0 case. The main results are as follows: (1) β is found to have a significant effect on the bimodal bubble volume probability density function inside the swirl chamber. In addition, the total resolved interfacial area of the near-orifice liquid film increases with β. (2) At the representative value of β = 0.2, the exact bubble size at the inlet is observed to have only a minor effect on the swirl chamber flow and liquid film characteristics. (3) The bubble-free (β = 0) and bubbly (β > 0) flows differ in terms of effective gas core diameter, core intermittency features, and spray uniformity. The quantitative analysis implies that bubble inclusion at the inlet affects the global liquid film characteristics with relevance to atomization.

The effect velocity and direction of swirling air flow on the formation spray downstream of pressure swirl nozzle

One of the methods for generation a finely dispersed spray for pilot nozzles, used in modern aircraft engines widely, is co-rotating or counter-rotating directed relative to the direction of movement of the fuel film, air flow swirl. Therefore, it is interesting to investigate the shape of the liquid film and its disintegration at low fuel consumption. In the present work, the influence of the speed and direction of the air flow on the main characteristics determining the dispersion and shape of the spray pattern downstream the pressure swirl nozzle was investigated. The mathematical model of the film motion in a curvilinear coordinate system associated with the film surface to obtain the basic characteristics of the spray was used. The main attention was paid to the spraying characteristics in the initial phase of the formation of the spray at the calculations carried out according to this model. Calculations showed that the swirling air flow helps to open the entire cone spray angle, however, the initial angle of the spray is smaller than in the absence of air flow near the nozzle with opposite swirling,. The work also showed that swirling air flow reduces the thickness of the fuel film in the immediate vicinity of the nozzle almost the same, regardless of the direction of its rotation. The effect of the swirling air flow on the average sizes of atomized fuel droplets based on the calculation of the thickness of the fuel film, its speed and the cone angle of the spray data were obtained. The results are compared with the available experimental date.

Experimental research on the influence of aviation fuel temperature on swirl nozzle velocity

The purpose of this research was to explore the effects of various fuel temperatures on the injection velocity of swirl nozzles. A phase Doppler particle analyzer (PDPA) and particle image velocimetry (PIV) were used to measure the velocity of fuel sprayed by an injector into the normal atmosphere. The flow characteristics and velocities of the droplet were studied at low to high temperatures (−40 °C to 80 °C). The results showed that: 1) the flow number (FN) and mass flow of the swirl nozzle decreased with the increase of the fuel temperature under the same supply pressure; 2) the existence of “pressure critical point” and “temperature critical point” makes the nozzle flow pattern and spray pattern change, thereby changing the distribution of droplet velocity and particle size in space; 3) the PDPA measurement results showed that under the same supply pressure, a larger droplet velocity on the fuel mist cone and a smaller Sauter Mean Diamter (SMD) were measured at high temperature of fuel.

Sound Production due to Swirl–Nozzle Interaction: Model-Based Analysis of Experiments

Indirect noise due to the interaction of flow inhomogeneities with a choked nozzle is an important cause of combustion instability in solid rocket motors and is believed to be important in aircraft engines. A previously published experiment (Kings, N., and Bake, F., “Indirect Combustion Noise: Noise Generation by Accelerated Vorticity in a Nozzle Flow,” International Journal of Spray and Combustion Dynamics, Vol. 2, No. 3, 2010, pp. 253–266.) demonstrated that interaction of a nozzle with time-dependent axial swirl can also be a source of sound. This axial swirl was generated by intermittent tangential mass injection upstream from a choked nozzle in a so-called vortex wave generator. The present work discusses the impact of swirl–nozzle interaction in this experiment on the acoustic waves detected downstream of the nozzle. The main source of sound appears to be the reduction in mass flux through the choked nozzle, which depends quadratically on the swirl number. This effect is quantitatively predicted by a quasi-steady and quasi-cylindrical analytical model. The model, combined with empirical data for the decay of axial swirl in pipe flows, predicts the observed influence of the distance between the vortex wave generator and the nozzle. The findings presented here contradict the hypothesis found in the literature, which presumes that sound production in the aforementioned experiment is due to the acceleration of vorticity waves through the nozzle.

Prediction of the Initial Drop Size and Velocity Distribution in the Cold Cryogenic Spray

The polydispersed nature of the spray is captured through the use of probability density functions based on the maximum entropy method to stand for the complete atomization characteristics of spray dynamics. The droplet and velocity size distributions are practical tools for the analysis of sprays cooling. The special benefit of the model is a Eulerian based which is less computationally intensive when compared to models that are based on the Lagrangian approach that tracks droplet parcel. The accuracy of using Lagrangian approach in polydispersed phase is always accurately less than Eulerian approach because it depends on the number of parcels while in Eulerian approach it depends on the proposed continuous distribution function. The main intent of the current work is to evaluate the capability of using the model for the initial predictions of the droplet size and velocity distribution for liquid nitrogen spray of solid-cone pressure swirl nozzle. The use of liquid injection pressure cases of up to 0.6MPa and spray cone angles of just 30◦ from three different sets of experimental data. The results being characterized are spray drop size distribution, liquid volume fraction and spray cone angle values. The unsteady analyses of the effect of injection pressure are studied on the cryogenic liquid nitrogen. The numerical results show that the maximum entropy method applies to liquid cryogenic spray and indicates that the model reacts correctly to changes in different injection pressures. Comparisons are also made with measured drop size distribution data that are reasonably captured and the spray cone angle is found to be in good agreement during initial and far-field spray angles.

Influence of asymmetric flame transfer functions on can-to-can acoustic interactions between two lean-premixed combustors

In can-annular gas turbines, low-frequency thermoacoustic instabilities can arise from cross-talk interactions between neighboring combustors upstream of the first-stage turbine nozzles. In this experimental study, we investigate the influence of non-identical flame transfer functions (FTFs) between adjacent combustors on the development of self-excited thermoacoustic oscillations. To create different FTFs, we use five different swirl nozzles, one with high swirl (HS) and four with low swirl (LS), all with different porosities. We find that, compared with the LS FTFs, the HS FTF exhibits a smaller and flatter gain as well as a smaller phase difference. We attribute this behavior to differences in the flame structure and the stabilization mechanisms, namely an inner shear layer-stabilized diverging front in the HS case versus a detached reaction zone in the presence of a central jet with an outer swirl flow in the LS cases. Using two tunable lean-premixed combustors connected via a cross-talk section, we show that (i) symmetric FTF combinations (HS + HS or LS + LS) produce in-phase interactions, leading to push-push modes, but that (ii) asymmetric FTF combinations (HS + LS) produce out-of-phase interactions, leading to push-pull modes. Phase-resolved visualization of the asymmetric cases reveals that the inner shear layer-stabilized HS flame exhibits large angle fluctuations, whereas the aerodynamically stabilized LS flame is characterized by the periodic emergence of a bow-shaped front and an oval structure. For all the conditions tested, we find that asymmetry in the FTFs leads to either (i) a completely stable state with negligible amplitude or (ii) a mildly unstable state with an amplitude lower than that of the equivalent symmetric cases. These findings highlight the potential of using asymmetric FTFs for passive control of cross-talk-driven thermoacoustic instabilities in can-annular combustors.

Smoke based visualization of turbulent swirl jet flow

The present study deals with the visualization of turbulent jet flow from swirl nozzles. A low-cost turbulent jet facility developed was utilized for all the experiments. Two types of vanes are considered for the present study viz. flat vanes and airfoil-shaped vanes. All the vanes are fixed to a circular hollow hub at the center. The flow is visualized by seeding smoke from incense material as tracer particles. A solid-state laser with 405nm wavelength is used for the illumination via a glass rod thereby producing a laser sheet. Cross-sectional visualizations are made at various axial downstream locations. The effect of the vane angle has a greater influence on the production of jet flow vortices due to swirlers. The present study reveals that the increase in the vane angle increases the production of vortices.

Experimental Study on Coaxial Swirling Flows

The experimental investigation of a swirling jet and an annular swirling stream issuing from coaxial cylinders is presented. The objective is to contribute to the research on the combined flow field close to vortex breakdown conditions. The two swirling water streams are interacting in the extension of the outer cylinder. Swirl is generated by two rotating impellers, located in the inner tube and the annular duct, just before the merging of the two streams. Controlled flow parameters comprise the flow rates of the streams and the angular velocities of the impellers. The flow field is monitored by means of Stereoscopic 3D-PIV, providing the velocity components on an axial, central plane. Four typical test cases were investigated comprising four combinations of inlet conditions. Two dimensionless numbers were utilized to interpret the experimental results, a modified Rossby number and the velocity ratio ζ, along with the Reynolds numbers of the internal and annular stream, respectively. The most important coherent structure developed, is a recirculation region formed downstream of the exit of the internal swirl nozzle. A bubble type vortex breakdown occurs when the appropriate flow conditions are applied. The alterations of the flow field were discussed with respect to the changes of the inlet conditions. The flowrates of the two streams and the combined swirl strength applied through the rotating impellers appear to be crucial for the onset of the vortex breakdown. Comparisons were drawn with previous work.

Influence of orifice geometry on atomization characteristics of pressure swirl atomizer.

The spray characteristics of the pressure swirl nozzle are experimentally studied using particle dynamics analysis (PDA) and high-speed photography system in this paper, specifically focusing on the dependence of geometrical dimensions of orifice on the spray SMD, velocity magnitude and droplet distribution, and the spray cone angle. It is indicated that the increase of orifice diameter makes the initial swirling velocity lower and the spray liquid film thicker. When the spray cone is fully expanded, the flow rate of 900 μm orifice diameter nozzle increases by 30-40% and the SMD of 900 μm orifice diameter nozzle increases by 8.5% compared with that of 700 μm orifice diameter nozzle. According to the experimental conditions, the relationship between Re and spray angle was calculated as θ = 29.97*Re0.087, ignoring the factors that had little influence on spray angle. The decrease of the orifice length makes the distance of gas-medium shearing action shorten so that thinner oil film near wall cannot be formed by the extrusion of air core, leading to the swirling intensity reducing and the suction effect weakened. The spray cone angle of the 450 μm orifice length atomizer is about 5° smaller than the nozzle of 500 μm orifice length, and more small SMD droplets are not sucked, resulting in the distribution range of spray SMD declining.

Spillback nozzle characterization using pulsating LED shadowgraphy

The atomization characteristics of a commercial spillback hydraulic nozzle were investigated in terms of droplet sizes, velocities, trajectories and spray angles. To replicate the fuel spray of a commercial auxiliary marine boiler, an experimental setup was designed and used to atomize water-glycerol solutions with various viscosities. Droplet size, velocity, position and shape were obtained with a novel pulsating LED backlight imaging system that employs a CCD camera to capture pairs of frames with a delay as short as 1 μs from one another. The overall spray characteristics of the nozzle and distributions for droplet size and velocity were examined for the operating conditions currently used in the burner and by varying three parameters: pressure drop, flow rate through the nozzle and liquid viscosity. Results include droplet size and velocity distributions in different spray regions, a description of the influence of each varying parameter on global indexes such as the Sauter Mean Diameter and a comparison with traditional correlations for pressure swirl nozzles.

Effect of Viscosities on the Spray Characteristics of Pressure Swirl Nozzle

The effects of medium viscosity on the spray flow rate, spray Sauter Mean Diameter, droplet velocity and spray cone angle of pressure swirl nozzles are investigated by making use of the particle dynamics analysis system and high-speed photographic system. Based on the axial and radial distribution characteristics of Sauter Mean Diameter and droplet velocity, the water-glycerol mixture is used to simulate medium with a wide range of viscosities. It is found that with the increase of viscosity, the turbulence of the medium flow and the swirling effect is weakened, and the rated pressure becomes larger and the spray flow rate increases. Spray Sauter Mean Diameter and droplet axial velocity becomes larger, while the spray cone angle decreases. The development of the axial velocity distribution of spray cone is characterized by the radial and axial position parameters. The area of the large-droplet region on both sides of spray cone becomes larger, and the area of small-droplet region near the axis becomes smaller.

Experimental research on spray evaporative cooling system applied to air-cooled chiller condenser

This study investigates the performance of a refrigeration unit by applying a spray evaporative cooling system to an air-cooled chiller. Experiments are conducted in the temperate monsoon climate region by combining the spray evaporative cooling system with the air-cooled chiller. Moreover, this study adopts a new gas–liquid two-phase swirl nozzle, which is independently developed and characterized by high atomization quality and low energy consumption. The experimental results show that the power of the compressor is reduced, while the COP (Coefficient of Performance) of the air-cooled chiller increases by 4%–8% after the intervention of the spray evaporative cooling system. The spray cooling system contributes to an electricity savings of 2.37%–13.53% in air-cooled chiller systems. If a more reasonable spray evaporative cooling mode is used, such as controlling the water flows according to the cooling capacity and arranging the nozzle positions and angles reasonably, the COP of the air-cooled unit will be further improved.

Computational and experimental investigation of a swirl nozzle for viscous fluids

Highly viscous flow in a large-scale pressure-swirl atomizer is studied by (1) 3d scale-resolving large-eddy simulations and volume-of-fluid method, and (2) experiments based on laser-Doppler anemometry, imaging techniques and pressure measurements. Here, a low Reynolds number regime (600 ≤ Re ≤ 910) is investigated by varying the mass flow rate of the water-glycerol mixture. The aim of the study is to perform a comprehensive comparison between the simulations and experiments at a parameter range and nozzle geometry relevant for biomass based fuels. We report the inner-nozzle velocity profiles noting good agreement for mean velocities inside the swirl chamber between the simulations and the experiments. Consistent with the earlier work (Laurila et al., 2019), the simulations indicate the flow mode to be laminar with weak or non-existent gaseous core inside the swirl chamber. As revealed by both approaches, liquid film shapes after the nozzle discharge orifice are qualitatively similar, of hollow cone type, and highly unstable. Both approaches indicate linear scaling of the liquid film velocity with the inlet Reynolds number and discharge coefficients to be in the range 0.57–0.64. The experimentally measured mean opening angles are reported to be 45–62∘, while the numerical counterparts show reasonable correspondence with the experiments. The results demonstrate the predictive ability of the present numerical method in swirl injector analysis.